1. Technical of the Invention
The present invention relates to a device and method for driving an EL device, and more particularly to a device and method for driving an EL device having a circuit configuration with an output transistor and a diode connected in parallel.
2. Related Art
An EL device (Electroluminescence device) has a structure like a condenser of one kind in which an illuminant having a special fluorescent material dispersed and contained in a dielectric material is sandwiched between two sheets of electrodes. When an AC voltage is applied between two sheets of electrodes for this EL device, an AC electric field is applied on the fluorescent material of the illuminant, so that the fluorescent material emits a light and the EL device is lightened. Generally, it is well known that the luminance and the luminance half life of the EL device depend on the magnitude and frequency of the applied voltage. In order to assure that this EL device has a longer life of the luminance, it is required that the AC voltage without superposition of direct current, i.e., the AC voltage having the same amplitude in positive and negative regions maybe applied between both electrodes of the EL device.
Generally, when controlling a plurality of EL devices, a drive circuit for the EL device is configured, as shown in FIG. 13 or 14. In FIG. 13, the EL devices (L1, L2, . . . , Ln) are connected in series to the relay switches (S1, S2, . . . , Sn), respectively, and to an AC power supply 39 for supplying an AC voltage of sinusoidal wave without superposition of direct current to make up a closed circuit. FIG. 14 shows a circuit configuration in which the TRIACs (T1, T2, . . . , Tn) are employed instead of the relay switches (S1, S2, . . . , Sn) in FIG. 13. The relay switches and the TRIACs can turn on or off the alternating current.
However, since the switches such as relay switches and TRIACs are large in size, the related circuit configuration is unsuitable for the EL devices mostly used to contribute to the miniaturization of the device, making use of the advantages of thin type and high luminance, and remarkably unfavorable particularly when a number of EL devices are driven.
To form an AC power supply circuit as a semiconductor integrated circuit, a circuit configuration as shown in FIG. 15, for example, is considered. In FIG. 15, both electrodes of an EL device L1 are connected to one electrodes of bipolar transistors (31 to 34), the other electrodes of bipolar transistors (31 to 34) being connected to a DC power supply and a ground potential point. The transistors (31, 33) connected to the DC power supply and the transistors (32, 34) connected to the ground potential point are turned on and off alternately to apply an AC voltage on the EL device. However, the AC voltages (Vx, Vy) applied to both electrodes of the EL device have a rectangular waveform, as shown in FIG. 16. If the EL device is driven by the AC voltage of this rectangular wave, the load on the EL device is increased owing to a sudden change in the applied voltage, unfavorably resulting in a shorter life of the EL device.